1. Field of the Invention
The present invention relates to an image recording method which is designed so that an electrostatic latent image of high resolution is formed on an insulating layer by utilizing an electric discharge, field emission, etc.
2. Description of the Related Art
As Japanese Patent Application No. 63-121592 discloses, a method wherein a photosensitive member consisting of a conductive layer and a photoconductive layer, which are successively formed on a substrate in the mentioned order, is disposed in close proximity and in opposing relation to a charge retaining medium consisting of a conductive layer and an insulating layer, which are successively formed on a substrate in the mentioned order, and exposure is carried out with a voltage being applied between the two conductive layers, thereby inducing a corona discharge between the photosensitive member and the charge retaining medium, and thus recording an image on the charge retaining medium and also effecting reproduction. According to this method, it is possible to effect analog recording of extremely high resolution and also possible to maintain the electrostatic latent image on the charge retaining medium semipermanently.
In such image recording, a high-voltage pulse is applied between the respective electrodes of the photosensitive member and the charge retaining medium to record an image without using an optical shutter. Alternatively, with a DC voltage being applied between the two electrodes, an image is recorded using an optical shutter.
However, the method wherein a high-voltage pulse is applied without using an optical shutter suffers from the problem that the rise and fall of the pulse are likely to become less sharp, and if precise control is effected so as to make the rise and fall of the pulse sharp, the cost of the system becomes extremely high. The method that uses an optical shutter to effect image recording involves the problem that the number of parts increases, which similarly results in a rise in the cost.
Although the above-described image recording method is extremely superior in that it provides high resolution and enables semipermanent recording, the prior art suffers from the following problems: In a corona discharge, charged particles that are accelerated by an electric field collide with other molecules one after another, causing ionization in an avalanche manner, and as a result, an electric discharging state is attained. For this reason, the collision between particles is essential for attaining and maintaining a discharging state. As the result of the collision between charged particles, the direction of motion of the particles is not always coincident with the direction of the electric field from the microscopic viewpoint, and the particles are therefore adsorbed on the charge retaining medium with a certain degree of spread, thus causing the resolution to be deteriorated.
Generally speaking, when a continuous tone image is to be expressed, in some cases it is difficult to obtain the desired output unless recording is effected using halftone dots. Devices that are commonly employed as hard copy devices, for example, ink-jet printers, thermal transfer printers, etc., are only capable of binary expression, that is, black-and-white expression, and these devices necessitate formation of halftone dots when a photographic tone image is desired to output.
In the dot forming process, the size of dots are changed in accordance with the image density without changing the pitch, thereby expressing a continuous tone.
However, an overwhelming majority of printers which are used as hard copy devices are only capable of black-and-white expression, and it is necessary in order to output a photographic tone image to carry out dot forming process, as described above. In domestic or amateur use, however, dot forming process must be executed using a personal computer or a built-in CPU, which is incorporated in a hard copy device. For a computer, the load of forming dots is extremely heavy in terms of both hardware and software, which results in a rise in the cost. In the field of printing, formation of dots is performed in order to obtain a precise image and this needs a signal processing for dot formation in a color scanner, so that the load on the system is extremely heavy and the cost is increased considerably.
The above-described image recording method further involves the problem that, if the voltage across the gap between the photosensitive member and the charge retaining medium exceeds a voltage at which Paschen discharge is initiated, a predetermined potential is undesirably generated on the charge retaining medium even in a case where no exposure is carried out, and this potential constitutes a background, which results in a lowering in the contrast.
It is an object of the present invention to provide an image recording method which enables a high-voltage pulse to be obtained without the need for precise control and which has no need to employ an optical shutter.
It is another object of the present invention to provide an image recording method which is designed so that the resolution is improved.
It is still another object of the present invention to provide an image recording method which reduces the load of dot formation and enables a continuous tone image to be readily recorded.
It is a further object of the present invention to provide an image recording method wherein an electric charge which is opposite in polarity to an image forming charge is formed in advance of the image exposure, thereby effecting precharging, and thus enabling the contrast to be improved.